[time-nuts] reference oscillator input circuit

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Dec 8 13:57:25 EST 2010


One caveat with driving a CMOS gate input is that if it is overdriven so 
that the input protection circuit diodes conduct then the output jitter 
may increase substantially.
At least this appears to happen when overdriving 74HC04 inverters.
Once the input diodes conduct (5MHz sine wave input) the output jitter 
is easily measured with a 5370A/B.
Just below conduction the output jitter appears to be buried in the 
5370A/B noise.

Bruce

Rick Karlquist wrote:
> The document cited is full of plausible sounding but misleading
> information, if you want really low jitter.  This type
> of oversimplification of the problem was present when the
> HP53131 series counters were designed, with the result that
> the Allan deviation of an external 10 MHz reference is degraded
> to only a part in 10^11 at 1 second.
>
> Rick Karlquist
>
>
> jimlux wrote:
>    
>> John Ackermann N8UR wrote:
>>      
>>> Wenzel has some discussion and circuits at:
>>> http://www.wenzel.com/documents/waveform.html.
>>>
>>> The Shera GPSDO made clever use of the input circuit of a 74HCT4046 PLL
>>> chip for squaring.
>>>
>>> John
>>> ----
>>>
>>>        
>> Doh... I was thinking that there was some vendor with ap notes on stuff
>> like this, and I couldn't recall who it was, and I'm actually looking at
>> a Wenzel OCXO sitting here on the desk.
>>
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>>      
>
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